The present invention generally relates to taps and threaded workpieces, and more particularly to producing correctly aligned threads in various metals, plastics, etc. within existing smooth bores. The parent application to the present application concerns self-tapping and self-aligning thread-replacement inserts.
When creating new threads in an existing smooth bore by means of a tap, it is obviously desirable that the new threads be correctly aligned to the axis of the bore hole. For example, if the tap enters the bore crookedly, it may bind or break, and, if it does not break but does produce crooked threads, then the crooked threads may cause misalignment of the threaded bore with the fastener and with the part secured by the fastener to the workpiece. The present invention addresses this alignment problem by modifying existing tap designs so that a pilot may be attached to and detached from the tap, to ensure that: 1) the tap enters the smooth bore straight; 2) the tap creates pilot threads accurately aligned to the smooth bore's axis; and 3) once the pilot threads have been created, that the pilot may be detached from the tap, and the tap reinserted into the newly formed pilot threads so that the smooth bore may be tapped fully through or to its bottom.
In machine shop and in industrial maintenance shop settings, the accurate tapping of holes is not normally problematic, in that power equipment such as mills and drill presses can be used to secure and position the workpiece, to properly align the tap to the workpiece by means of a chuck, collet, etc., and even to rotate the tap. However, in field settings, outside such well equipped shops, the tapping operation is usually done by hand, using a tap handle or similar device to rotate the tap. In such field settings, correct alignment of the tap to the smooth bore is often problematic. In order to start the tap straight and maintain its alignment, the operator normally relies on his eye alone, or sometimes uses assistants (“spotters”) stationed to one or more sides of the workpiece to aid with the alignment of the tap. Known ways of increasing the chance that the tap's axis will match that of the smooth bore include: 1) hand-held tap guides, of a bushing-type design, which are positioned over the bore and held in place with one hand, while the operator's other hand, or an assistant, rotates the tap handle; 2) Bushing-type guides positioned over the bore hole and clamped or otherwise attached to the surface of the workpiece; 3) Taps with integral guides, whose integral guide is inserted into the bore hole to guide the tap, after which, especially when it is a blind hole that is being tapped, a second tap of the same thread pitch but without an integral guide is used to complete the tapping operation.
The disadvantages of the currently known tapping aids are: 1) Hand-held tap guides are only as accurate as the operator's skill, attentiveness, and site conditions allow. Larger diameter taps, which require high torque and larger tap handles to install, are unsuited to the use of hand-held tap guides. 2) Tap guide fixtures that attach to the workpiece are cumbersome to operate, are time-consuming to install, and are limited by space constraints and suitable attachment surfaces. 3) Both 1 and 2 above are ineffective on uneven or curved surfaces. 4) Taps with integral guides cannot create new threads all the way to the bottom of a “blind” hole, because the tap's pilot area bottoms the tap out before the tapping operation is complete, such that a second tap, without a guide, is needed to finish the job. A disadvantage of conventional (non-guided) taps is that, due to the difficulty of starting them straight, conventional taps are manufactured in three versions: a starter (taper) tap, an intermediate (plug) tap, and a finishing (bottoming) tap.
A need therefore exists, particularly under field conditions, for a tap which consistently aligns itself to the axis of the bore hole, regardless of the workpiece's shape or surface condition; which does so without the aid of any hand-held apparatus or apparatus attached to the workpiece and without the limitations of an integral-guide tap, and which eliminates the risks associated with measuring alignment “by eye.”